Compressor terminal fault interruption method and apparatus

ABSTRACT

A compressor terminal fault interruption method and interrupter for disconnecting power to a compressor terminal when terminal venting failure is imminent including a current sensing circuit for sensing current provided to the terminal by a power source and outputting a sensed signal representing the current provided to the terminal and a control circuit. The control circuit includes a first circuit for outputting a reference signal representing input current much higher than locked rotor current, a second circuit connected to the current sensing circuit and the first circuit for comparing the sensed signal to the reference signal, and a third circuit connected to the second circuit for disconnecting power to the terminal when the sensed signal exceeds the reference signal, thereby preventing excessive current from reaching the compressor terminal.

FIELD OF THE INVENTION

[0001] The present invention relates generally to an over-currentprotection method and circuit, and more particularly to a method andcircuit for disconnecting power to a motor for a hermetic compressorupon detecting excess current.

BACKGROUND OF THE INVENTION

[0002] Refrigeration systems, such as residential refrigerators, useelectric motor powered hermetic compressors which compress the systemrefrigerant according to principles well known in the art. Under certainconditions, the compressor motor in a system can enter a fault modewherein the power lines to the compressor input terminals carryexcessively high current. This high current condition may result in aphenomenon commonly referred to as “terminal venting”.

[0003] Terminal venting is generally characterized by a separation ofthe metallic compressor input terminal pins from the surroundinginsulating material in which the pins are mounted. This can occur ifexcessively high current is supplied to the terminals for sufficienttime to destroy the glass insulating seal. The problem is exacerbated bythe different thermal expansion coefficients of the pins and theinsulating material thereby causing destructive tensile stresses in theglass. The end result of such a failure is damage to the hermetic sealof the compressor terminal and, in some situations, the uncontrolledrelease of refrigerant gas.

[0004] Many compressor manufacturers incorporate mechanical safeguardsinto their compressor designs to reduce the likelihood and/or theeffects of terminal venting. Some conventional compressors employ robustinsulating materials with high temperature breakdown characteristics.Other compressors include covers which enclose the compressor terminals.

[0005] Conventional fuse-based interrupt circuits for similarapplications do not adequately prevent terminal venting because suchcircuits are typically triggered by a prolonged presence of currentlevels substantially lower than the current levels associated withterminal venting. For example, when the compressor rotor becomes locked,the compressor motor draws high current (commonly referred to as “lockedrotor current”) such as 20 amps, for example, but not nearly as high asthe current associated with terminal venting, which is typically inexcess of 50 amps. Conventional interrupt circuits interrupt power tothe compressor to protect the motor coils when the current draw of thecompressor motor is in the range of locked rotor currents, and issustained for a sufficiently long period of time. While theexceptionally high current associated with terminal venting wouldtypically trigger a conventional interrupt circuit the relatively slowresponse time of such circuits requires a prolonged application of thishigh current. Thus, damage to the compressor terminals may occur longbefore a conventional interrupt circuit is triggered.

SUMMARY OF THE INVENTION

[0006] It has been determined that if the temperature differentialbetween the pin and glass exceeds a given threshold for a particularterminal, the resulting tensile stresses in the glass will cause failureof the pin-to-glass seal and result in terminal venting. In accordancewith the method of the present invention and the particular exemplarycircuit implementation shown, the current flowing through the terminalis detected. If the detected current exceeds a threshold level that,unless substantially. immediately terminated, will cause the pin/glasstemperature differential to rapidly exceed a threshold level resultingin glass stresses that will cause the pin-to-glass failure and terminalventing, power through the terminal is immediately terminated. Thethreshold current level is much higher than locked rotor current for thecompressor motor, preferably at least two times the locked rotorcurrent. It has been found that once the pin current exceeds a giventhreshold for a particular terminal, that even if the current rise is nohigher, the pin and glass temperatures continue to rise and thepin/glass temperature differential where failure of the pin-to-glassseal occurs will rapidly be reached. Therefore, the threshold currentselected for a particular terminal must be lower than that whichcorrelates to simultaneous pin and glass temperatures at the failurelevel.

[0007] The present invention can be implemented by an exemplaryprotection circuit connected in series between the power lines andterminal of the compressor which detects the presence of a motor faultor other over-current condition and disconnects power to the terminal toprevent terminal venting due to this condition. The circuit generallyincludes a line-connected power supply for powering the circuit, acurrent sensor for sensing the current drawn by the compressor motor anda control circuit for disconnecting power to the motor when a fault isdetected. The circuit may include an audible or visual alarm to indicatethe presence of a fault. Additionally, since the present protectioncircuit is connected in-line with the power connections to thecompressor and external of the compressor housing, existing compressorsmay readily be retrofitted to obtain the protection against terminalventing provided by the present invention.

[0008] The method and circuit of the present invention protect thecompressor terminals, as opposed to the motor coils, by quicklydisconnecting power to the compressor, but only upon detection ofexceptionally high current levels. This high threshold permitssimultaneous operation of conventional interrupt circuits and prevents“nuisance triggering” as a result of the large current demands at motorstart-up or current noise spikes that may occur during operation. Whilethe current threshold of the present protection circuit is quite highrelative to the locked rotor current, damage to the compressor terminalsis nonetheless prevented because the response time of the circuit issubstantially faster than conventional interrupt circuits. For example,current is terminated within 20 milliseconds of detecting the presetcurrent threshold. Thus. the exceptionally high current is removed fromthe compressor terminals before the temperature of the terminal pincauses damage to the pin-to-glass seal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other features of the present invention will becomemore apparent and the invention will be better understood uponconsideration of the following description of the accompanying drawingswherein:

[0010]FIG. 1 is a block diagram of a portion of a refrigeration systemwith an over-current protection circuit according to the presentinvention.

[0011]FIG. 2 is a cross-sectional view of a compressor showing thecompressor input terminals.

[0012]FIG. 3 is a schematic diagram of an over-current protectioncircuit according to an exemplary embodiment of the present invention.

[0013]FIG. 4 is a graphical representation of the temperatures of thepin and glass when high current is applied across the pin.

[0014]FIG. 5 is a graphical representation of the differentialtemperature of the pin and glass of the hermetic terminal for differentcurrents.

[0015]FIG. 6 is a further graphical representation of the differentialtemperature of the pin and glass of the hermetic terminal when differentcurrents are applied.

[0016]FIG. 7 is a graphical representation of the maximum principaltensile stress in the hermetic terminal when different currents areapplied for seven seconds.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0017] The embodiment of the invention described herein is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed. Rather, the embodiment selected for description has beenchosen to enable one skilled in the art to practice the invention.

[0018] Referring now to FIG. 1, in a typical refrigeration system 10,power is supplied from a power source 12, such as a wall outlet, to acompressor motor 14 which drives a compressor 16. The present inventionmay be applied to any hermetic compressor used, for example, in airconditioning and refrigeration applications such as the numerous modelsof compressors commercially available from the assignee of the presentapplication, Tecumseh Products Company. For example, compressor 16 couldbe of the type disclosed in U.S. Pat. No. 5,199,898 which is assigned tothe assignee of the present invention and is expressly incorporatedherein by reference. According to the present invention, a protectioncircuit 100 is connected between power source 12 and terminal assembly18 for the compressor 16 and motor 14 to detect an excessive currentdraw and disconnect power in response thereto. The power connectionsshown in FIG. 1 include a high line, a common line, and a ground line.As will be explained further below, while protection circuit 100 will bedescribed as disconnecting the common line to compressor motor 14 upondetecting an over-current condition, protection circuit 100 couldreadily be adapted to disconnect either the high signal or the commonsignal provided to motor 14.

[0019] Referring now to FIG. 2, the configuration of the compressorinput terminals is shown. Compressor 16 generally includes a hermetichousing 17 and a terminal assembly 18 in which are mounted threeterminals (only two shown). Terminal 20 carries the power high line frompower source 12 to compressor motor 14 through wire 22, connector 24,and pin 26. Similarly terminal 28 carries the common line from powersource 12 through wire 30, connector 32, and pin 34. Terminal pins 26and 34 are mounted within housing 18 and glass insulating material 36according to principles well known in the art. A terminal ventingcondition is characterized by separation between any of terminal pins26, 34, or the neutral terminal pin (not shown) from glass insulatingmaterial 36, potentially resulting in an uncontrolled release ofrefrigerant from compressor 16. The excessive current drawn which maylead to such a failure is prevented from reaching compressor 16 byover-protection current 100 as described below.

[0020]FIG. 3 shows a protection circuit 100 which can be used toimplement the method and apparatus of the present invention. Circuit 100includes a regulator circuit 102 to establish a fixed DC voltage forcomparing to a voltage representing the current drawn by compressormotor 14, a current sensing circuit 104 for deriving this representativevoltage, and a control circuit 106 for disconnecting power to compressor16 as will be further described below. Regulator circuit 102 includes atransformer Ti, shown as a 36 volt device, the primary side of which isconnected to 117 VAC power from power source 12 (FIG. 1). The outputsignal from the secondary side of transformer T1 is rectified by diodesD1, D2 to produce a 24 VDC signal. This 24 VDC signal is used toenergize relay RYI as will be further described below. The 24 VDC signalis filtered by capacitor C1 and passed through a 5 volt regulator U1 toproduce a 5 VDC signal at the output of regulator circuit 102. Thissignal is further filtered by capacitor C2, and passed through a voltagedivider network in control circuit 106 including resistors R2, R3.

[0021] The output node 108 of voltage divider R2, R3 is the referencevoltage used to set a maximum threshold for the acceptable currentprovided through terminal 18 to compressor motor 14. As indicated above,this threshold reference voltage is set such that the increased currentdraw associated with motor start-up or other typical operatingconditions does not result in activation of circuit 100. Moreover, thereference voltage is set such that triggering of circuit 100 occurs onlyupon detection of motor 14 current substantially higher than lockedrotor current. The threshold current level causing activation of thedisconnect circuit is that current which, unless substantiallyimmediately terminated, will cause the pin/glass temperaturedifferential to rapidly exceed a threshold level resulting in glassstresses that will cause pin-to-glass failure and terminal venting. Thisthreshold current is much higher than locked rotor current for thecompressor motor, for example, at least two times the locked rotorcurrent. For a typical terminal, such as a No. 40387 terminal providedon a TP or TW series compressor having a 200-300 watt motor availablefrom Tecumseh Products Company, the threshold current at 115 volts is 52amps.

[0022] As indicated earlier, it has been found that once the pin currentexceeds a given threshold for a particular terminal, even if the currentrises no higher or is terminated, the pin and glass temperaturescontinue to rise and the pin/glass temperature differential wherefailure of the pin-to-glass seal occurs will rapidly be reached.Accordingly, the threshold current selected for a particular terminalmust be lower than that which correlates to simultaneous pin and glasstemperatures at the failure level.

[0023] The 117 VAC high line is passed through current sensor CS1 ofcurrent sensing circuit 104 to compressor motor 14 (compressor terminal20). Current sensor CS1 is a conventional torroidal current sensor, andis connected to resistor R4 and rectifier D3, D4. Since small voltagechanges are produced by current sensor CS1 in response to currentchanges on the 117 VAC power line, Schottky diodes are used forrectifier D3, D4 to minimize the forward voltage drop incurred by theoutput voltage of current sensor CS1. As current through current sensorCS1 increases, the voltage at the output of rectifier D3, D4 alsoincreases. This signal is passed through resistor R5 and filtered byresistor R6 and capacitor C3. The filtered signal is connected to thepositive input of comparator U2A of control circuit 106. A diode D5 isconnected between the positive input of comparator U2A and ground toprotect comparator U2A in the event a large voltage is generated bycurrent sensor CS1. Specifically, if the voltage at the positive inputof comparator U2A exceeds the 6.2 voltage breakdown voltage of diode D5,diode D5 will reverse bias and conduct to ground, thereby protecting theremainder of circuit 100.

[0024] The negative input to comparator U2A is connected to thereference voltage at node 108 of voltage divider R2, R3. The output ofcomparator U2A is connected to pull up resistor R7 which is connected tothe 5 VDC output power from regulator circuit 102. The output ofcomparator U2A is also connected to Schottky diode D6 which isolatescomparator U2A from an AND gate U3. Both inputs of AND gate U3 areconnected together and connected to the filter including resistor R8 andcapacitor C4. A hysteresis resistor R9 is connected from the output ofAND gate U3 to the inputs. The output of AND gate U3 is also connectedto the negative input of comparator U2B, the positive input of which isconnected to the reference voltage at node 108 of voltage divider R2,R3. As will be further explained below, comparator U2B functions as aninverter.

[0025] The output of comparator U2B is pulled up by resistor R10 andconnected to the gate of transistor Q1. The drain of transistor Q1 isconnected to ground and the source is connected to the low side of thesolenoid coil of relay RY1. The high side of the solenoid coil isconnected to the 24 VDC signal from rectifier D1, D2 of regulatorcircuit 102. Relay RY1 is shown in its energized configuration whereinthe common line from power source 12 (FIG. 1) is passed through theswitch of RY1, terminal 28 of compressor 16, to compressor motor 14.

[0026] In operation, when excess current is drawn by motor 14 throughthe 117 VAC high line, current sensor CS1 produces an output voltagewhich is rectified by diodes D3, D4 and provided to the positive inputof comparator U2A after filtering by resistor R6 and capacitor C3. Ifthe voltage exceeds the reference voltage (from note 108 of voltagedivider R2, R3) at the negative input to comparator U2A, comparator U2Aoutputs a positive logic signal. Accordingly, a positive logic signal ispresent at both inputs to AND gate U3, causing a positive output. Thecombination of Schottky diode D6 and hysteresis resistor R9 latch theoutput of AND gate U3 in the logic high state. A logic high state istherefore present at the negative input to comparator U2B. Controlcircuit 106 is designed such that this signal exceeds the referencevoltage at the positive input to comparator U2B. Accordingly, comparatorU2B outputs a logic low signal disabling transistor Q1. The path toground for the solenoid coil of relay RY1 is thereby removed,de-energizing relay RY1 such that relay RY1 switches to an openposition.

[0027] When relay RY1 opens, power is disconnected to compressor motor14, and the current passing through current sensor CS1 quickly goes tozero. This rapid disconnect prevents the excessive current at terminals20, 28 (and the third terminal, not shown) from heating terminals 20. 28to a temperature resulting in terminal venting. As should be apparent toone skilled in the art, a relationship exists between the referencevoltage and the speed at which circuit 100 disconnects power tocompressor motor 14 (i.e., the response time). Since circuit 100 isdesigned to prevent damaging temperature levels at terminals 20, 28, thehigher the reference voltage is set, the faster the required responsetime. As a corollary, a slower response time may be used (requiring alonger duration high current condition) if a lower reference voltage isset. For the particular example described above, the time betweendetection of the threshold current and the energizing relay RY1 is 22milliseconds.

[0028] When power is disconnected to compressor motor 14 and currentsensor CS1 goes to zero, the positive input to comparator U2A fallsbelow the negative input (the reference voltage from voltage divider R2,R3), causing comparator U2A to output a logic low signal. As mentionedabove, however, the output of AND gate U3 remains in a logic high statesince Schottky diode D6 isolates the output of comparator U2A from theinputs to AND gate U3, and hysteresis R9 feeds back the logic highoutput of AND gate U3 to its inputs. Accordingly, once the referencevoltage is exceeded by the voltage representing the current sensed bycurrent sensor CS1, circuit 100 disables relay RY1 and maintains relayRY1 in a disabled state, thereby disconnecting power from compressormotor 14, until power is removed from circuit 100 and re-applied. Thus,when circuit 1000 disables compressor motor 14, compressor motor 14remains disabled until it is properly serviced.

[0029] Referring now to FIG. 4, there is provided a graphicalrepresentation of the pin and glass temperatures as a function of timewhen different currents are applied to terminal pin 26 or 34. As can beseen, the higher the pin current the more rapid the rise in pintemperature and concomitantly the temperature differential between thepin and glass. FIG. 5 illustrates this rise in pin/glass temperaturedifferential, and particularly for high current levels, such as 120amps, the temperature differential curve rises very sharply after onlyone second following initiation of the high current condition. FIG. 6 isa similar representation but includes additional current levels in amathematical simulation.

[0030]FIG. 7 shows graphically the rapid rise in maximum principalstresses in the glass as the temperature differential between the glassand pin increases. As is quite evident, the curve is substantiallyexponential thereby indicating that unless current is terminated at avery early time when the threshold current is detected, rapid heatingand failure of the pin-to-glass seal will occur.

[0031] Utilizing the data from FIG. 5, the following mathematical modelequation describing the process to prevent terminal venting wasobtained:

T=a ₁ i ^(b) ^(₁) t ⁴ +a ₂ i ^(b) ^(₂) t ³ +a ₃ i ^(b) ^(₃) t ²

[0032] In this equation, T is the differential temperature between thepin and the glass in degrees Celsius. i is current through the pin inamperes and t is the amount of time in seconds current has been applied.Once the maximum temperature differential between the pin and glass isdetermined for a particular terminal, the equation can be solved forcurrent in order to set the threshold level in circuit 100.

[0033] The experimental data to generate and validate the curvesdiscussed above was obtained by applying different levels of currentthrough the terminal and measuring the temperature of the glass and pin.The constants a₁, a₂, a₃, b₁, b₂, b₃ are derived from the curves and areused for the particular terminal construction tested. For theaforementioned terminal, the constants are as follows:

a ₁=1.079×10⁻⁴

a ₂=2.240×10⁻³

a ₃=1.4447×10⁻²

b ₁=1.8875

b ₂=1.8000

b ₃=1.7335

[0034] Maximum allowable glass stress and therefore maximum allowablepin-to-glass temperature differential is determined by measuring theelectrical isolation resistance of the pin-to-glass interface, whichwill be indicative of the glass stress. For example, if the maximumallowable stress for the particular terminal tested was chosen to be12,500 psi, this correlates to a pin/glass differential temperature ofabout 210° C. (FIG. 7). As can be seen in FIG. 5, this temperaturedifferential would be reached under 80 amp current conditions. Since apin-to-glass temperature differential of 55° C. results from twice thelocked rotor current of 40 amps. 52 amps was selected as the thresholdcurrent level for power interruption.

[0035] The data set forth above is only exemplary and applies to aparticular terminal. However, the same technique can be applied to anyterminal by taking temperature and isolation resistance measurements ata variety of pin currents until the maximum allowable pin/glasstemperature differential is attained.

[0036] Although a discrete circuit has been disclosed to perform themethod of the present invention, other implementations are obviouslypossible, such as implementation by way of a microprocessor programmedto respond to the current input signal and provide an output signal todisconnect the motor upon the threshold current level being sensed.Although relay RY1 is shown in the exemplary system as the device fordisconnecting power between the power source and the compressor motor14, this could be replaced by an normally open relay held closed by thecircuit, a latching relay, piezoelectric relay, bi-metal relay or solidstate relay-type device, such as an SCR, triac, FET, etc. It may also bedesirable to have the protection circuit 100 be non-resettable. Otherpotential implementations of the current sensor CS1 can be a transformerisolated hall effect device, GMR, etc., or other current sensing meanswell known in the art.

[0037] Although the present invention has been shown and described indetail, the same is to be taken by way of example only and not by way oflimitation. Numerous changes can be made to the embodiments describedabove without departing from the scope of the invention. For example,circuit 100 could readily be reconfigured to monitor the power demand ofcompressor motor 14 in terms of watts versus time by way of thetechnique disclosed in pending U.S. application Ser. No. 09/697.631filed Oct. 26, 2000, which application is expressly incorporated hereinby reference. This application is therefore intended to cover anyvariation, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

What is claimed is:
 1. A method of preventing damage to the terminal ofa hermetic compressor having a motor, said method comprising the stepsof: sensing current draw through the terminal; monitoring a signalrepresenting the sensed current draw; comparing the monitored signal toa reference signal corresponding to a current draw substantially greaterthan a current draw associated with a locked rotor condition of themotor; and rapidly disconnecting power to the terminal when themonitored signal exceeds the reference signal to prevent heating of thecompressor terminal to a level likely to cause terminal venting.
 2. Themethod of claim 1 wherein the reference signal corresponds to a currentdraw greater than twice the current draw associated with a locked rotorcondition of the motor.
 3. The method of claim 1 wherein the referencesignal corresponds to current draw that will subsequently heat theterminal to a level that the differential temperature between a pin andsurrounding glass of the terminal exceeds a level where stresses in theglass will cause failure of the pin/glass seal.
 4. The method of claim 1wherein the current draw is sensed externally of the compressor.
 5. Amethod of preventing damage to the terminal of a hermetic compressorhaving a motor, said method comprising the steps of: sensing power drawthrough the terminal; monitoring a signal representing the sensed powerdraw; comparing the monitored signal to a reference signal correspondingto a power draw substantially greater than the power draw associatedwith a locked rotor condition of the motor; and rapidly disconnectingpower to the compressor terminal when the monitored signal exceeds thereference signal to prevent heating of the compressor terminal to alevel likely to cause terminal venting.
 6. The method of claim 5 whereinthe reference signal corresponds to a power draw greater than twice thepower draw associated with a locked rotor condition of the motor.
 7. Themethod of claim 5 wherein the reference signal corresponds to power drawthat will subsequently heat the terminal to a level that thedifferential temperature between a pin and surrounding glass of theterminal exceeds a level where stresses in the glass will cause failureof the pin/glass seal.
 8. The method of claim 5 wherein the power drawis sensed externally of the compressor.
 9. A hermetic compressor havinga hermetically encased motor, a terminal assembly and a compressor faultinterruption circuit, said fault interruption circuit comprising: acurrent draw sensor; a reference signal source representing a currentdraw threshold level that is much higher than locked rotor current; acomparison circuit connected to receive inputs from said current drawsensor and said reference signal source; and a disconnect deviceconnected in series with and ahead of said terminal assembly andcontrolled by said comparison circuit.
 10. A fault interruption circuitfor disconnecting power to a compressor terminal under very high currentconditions to prevent damage to the terminal, the circuit including: acurrent sensing circuit disposed externally of said compressor forsensing input current provided to the terminal by a power source andoutputting a sensed signal representing said current; and a controlcircuit including a first circuit for outputting a reference signalrepresenting input current higher than locked rotor current, a secondcircuit connected to the current sensing circuit and the first circuitfor comparing the sensed signal to the reference signal, and a thirdcircuit connected to the second circuit for externally disconnectingpower to the terminal when the sensed signal exceeds the referencesignal.
 11. The circuit of claim 10 wherein the current sensing circuitincludes a current sensor coupled to a line carrying power from thepower source to the terminal, and a rectifier connected to the currentsensor for converting an output from the current sensor into a DCvoltage proportional to the sensed signal.
 12. The circuit of claim 10further including a regulator circuit connected to the power source, theregulator circuit including a regulator connected to the control circuitfor outputting a DC voltage, the first circuit including a voltagedivider network having an input connected to the regulator, the voltagedivider network producing the reference signal from the DC voltageoutput of the regulator.
 13. The circuit of claim 10 wherein the secondcircuit includes a comparator having a first input connected to thecurrent sensing circuit output for receiving the sensed signal, a secondinput connected to the first circuit of the control circuit forreceiving the reference signal, and an output for outputting a firstsignal when the sensed signal exceeds the reference signal.